This invention relates to improvements in a solid electrolyte, in a method of producing the solid electrolyte and in a fuel cell using the solid electrolyte, and more particularly to the solid electrolyte which is active to maintain high ionic conductivity at low temperatures and stabilized in ionic conductivity, the method of producing the solid electrolyte and the fuel cell using the solid electrolyte.
Recently, researches and developments have been positively proceeded on solid electrolytes because the solid electrolytes are safe from the viewpoint of liquid leak and specified ions being conducted, so that they are very effective as electronic materials of a variety of devices such as cells and gas sensors. Particularly, developments have been proceeded on ceramic solid electrolyte fuel cells called SOFC (solid oxide fuel cell). A fuel cell having a zirconia-based ceramic solid electrolyte has made an operational achievement in which power generation of several kW is maintained for several thousands hours. It is supposed that the SOFC is operated at high temperatures higher than 1000xc2x0 C., and therefore hydrocarbon fuels can be reformed inside the fuel cell (accomplishing so-called internal reforming) thereby obtaining a high combustion or conversion efficiency higher than 60%.
In general, the SOFC is composed of a solid electrolyte, an anode and a cathode. All such materials are required to be stable in oxidizing and reducing atmosphere, to have suitable ionic conductivity, and to have their thermal expansion coefficients close to each other. Additionally, the materials of the anode and the cathode are required to be so porous that gas is permeable. Further, the materials of the SOFC are desired to be high in strength and stiffness, to be inexpensive, to be operable at temperatures as low as possible (as basic requirements for electric conductive materials) from the viewpoint of safe during operation of the SOFC.
Presently, stabilized ZrO2 is in the mainstream of the materials of the solid electrolytes, in which oxide of bivalent alkaline earth metal such as CaO, MgO, Sc2O3 or rare earth oxide such as Y2O3 are used as a stabilizer. ZrO2 doped with CaO (oxide of alkaline earth metal) exhibits an ionic conduction characteristic value of 0.01 (xcexa9cm)xe2x88x921. Additionally, H. Tannenberger et al has reported in xe2x80x9cProc. Int""l Etude Piles Combust, 19-26 (1965)xe2x80x9d that the ionic conductivity of ZrO2 doped with one of Y2O3, Yb2O3, and Gd2O3 is around a range of from 1xc3x9710xe2x88x921 to 1xc3x9710xe2x88x922 S/cm at 800xc2x0 C., and decreases to a value lower than 2xc3x9710xe2x88x922 S/cm when temperature is below 650xc2x0 C.
Concerning zirconia stabilized by rare earth and alkaline earth compounds, they are disclosed in Japanese Patent Publication No. 57-50748 and Patent Publication No. 57-50749.
Additionally, stabilized bismuth oxide is also used as solid electrolyte. A high temperature phase (xcex4 phase) of Bi2O3 has a deficiency fluorite structure (Bi4O6xe2x96xa12 where xe2x96xa1 is vacancy) and low in activation energy for oxide ion movement thereby exhibiting a high oxide ion conductivity. The high temperature phase is stabilized also in a low temperature region by forming solid solution of rare earth oxide, thus exhibiting a high oxygen ion conductivity. T. Takahashi et al reports in xe2x80x9cJ. Appl. Electrochemistry, 5(3), 187-195(1975)xe2x80x9d that bismuth oxide stabilized by oxide of rare earth element, for example, (Bi2O3)1xe2x88x92x(Y2O3)x exhibits ionic conductivity characteristics of 0.1 (xcexa9cm)xe2x88x921 at 700xc2x0 C., 0.01 (xcexa9cm)xe2x88x921 at 500xc2x0 C. which are higher 10 to 100 times than stabilized zirconia.
Japanese Patent Publication No. 62-45191 recites that a mixture of stabilized bismuth and stabilized zirconium oxide exhibits an ionic conductivity of 0.1 (xcexa9cm)xe2x88x921 at 700xc2x0 C. Accordingly, it may be expected that a high ionic conductivity can be obtained in a temperature region lower than 1000xc2x0 C. However, bismuth oxide is reduced into bismuth in metal state under a reduction atmosphere thereby exhibiting electronic conductivity, and therefore it is difficult to directly use the mixture as solid electrolyte.
Additionally, ceria-based solid solution is also used as solid electrolyte. CeO2 has a fluorite-type cubic structure in a temperature ranging from loom temperature to melting point. Kudo and H. Obayashi et al reports in xe2x80x9cJ. Electrochem., Soc., 123[3] 416-419, (1976)xe2x80x9d that solid solution is formed in a wide temperature region by adding rare earth oxide or CaO to CeO2.
CeO2xe2x80x94Gd2O3-based solid electrolyte which is in the main stream compound of recent researches and developments is represented by Ce1xe2x88x92xGdxO2xe2x88x92x/2 in which vacancy of oxygen is formed. In such compound, the valency of Ce is changed and therefore cerium oxide is reduced into cerium in metal state under a reduction atmosphere similarly to bismuth oxide, thereby exhibiting electronic conductivity. Accordingly, it is difficult to directly use such compound as solid electrolyte.
As other materials usable in a low temperature region, attention has been paid on research and development of perovskite compound. This compound is composed of ABO3 having two ions (A and B) and has such examples as BaCe0.9Gd0.1O3, La0.8Sr0.1Ga0.8Mg0.2O3, CaAl0.7TiO3 and SrZr0.9Sc0.1O3. Additionally, La1xe2x88x92xSrx Ga1xe2x88x92yMgyO3 is reported by T. Ishihara et al reports in xe2x80x9cJ. Am. Chem. soc., 116, 3801-03 (1994) and by M. Feng and J. B. Goodenough reports in xe2x80x9cEur. J. Solid. State Inorg. Chem. t31, 663-672 (1994)xe2x80x9d.
However, such zirconia is low in ionic conductivity in a low temperature region, and electronic conductivity of bismuth oxide and ceria is in the reduction atmosphere. Accordingly, they are not suitable for solid electrolyte of fuel cell in a low temperature region. Additionally, although perovskite compound is high in ionic conductivity in a low temperature region as compared with other compounds, it is lowered in oxygen ion conductivity in such a low temperature region under Hall effect.
In the above-discussed fuel cells, power output of a single cell is limited to about 1V, and therefore it is required to obtain a high power output that a fuel cell takes a laminated structure including a plurality of single cells. Such a ceramic fuel cell having the laminated structure becomes large-sized, which makes it difficult to select structures (for example, tube-type or plate-type) of parts and to produce a large-sized fuel cell. A container such as a combustor main body of such a large-sized ceramic fuel cell requires to effectively use metal parts formed of ferrite stainless steel or the like from the economical view points. In order to effectively use metal, the fuel cell requires stabilized solid electrolyte materials which are active throughout a wide temperature region, for example, in a low temperature region (600 to 800xc2x0 C.) so as to have an ionic conductivity generally equal to that in a high temperature region higher than 1000xc2x0 C.
Additionally, solid electrolyte has crystal which is liable to break at temperatures around 650 xc2x0 C. Accordingly, it has been required to establish a technique for stabilizing crystal phase of solid electrolyte in a wide temperature region and to prevent solid electrolyte from lowering in strength at high temperatures. In this regard, Japanese Patent Provisional Publication No. 5-225820 discloses that AlO3 is added for the purpose of stabilizing crystal structure of the solid electrolyte.
It is, therefore, an object of the present invention to provide an improved solid electrolyte and an improved method of producing the solid electrolyte, which can overcome drawbacks encountered in conventional techniques in connection with solid electrolyte.
Another object of the present invention is to provide an improved solid electrolyte which is sufficiently active throughout a wide temperature region including a relatively low temperature of around 600xc2x0 C., and high in ionic conductivity and stabilized.
A further object of the present invention is to provide an improved solid electrolyte which is active in a low temperature range around 600xc2x0 C. and high in ionic conductivity, while being able to prevent Hall effect from decreasing thereby improving its transference number.
A still further object of the present invention is to provide an improved method of producing a solid electrolyte which is sufficiently active throughout a wide temperature region including a temperature range around 600xc2x0 C., and high in ionic conductivity and stabilized.
A still further object of the present invention is to provide an improved fuel cell including a solid electrolyte which is sufficiently active throughout a wide temperature region including a temperature range around 600xc2x0 C., and high in ionic conductivity and stabilized.
An aspect of the present invention resides in a solid electrolyte represented by the following formula:
La(1xe2x88x92xxe2x88x92y)LnxAyGa(1xe2x88x92z)BzO3xe2x88x920.5(x+y+z)
where Ln is rare earth element; A is at least one element selected from the group consisting of Sr, Ba and Ca; B is at least one of Mg and Zn; x is a number ranging from 0.05 to 0.15; y is a number ranging from 0.05 to 0.15; and z is a number ranging from 0.05 to 0.25.
Another aspect of the present invention resides in a method of producing a solid electrolyte, comprising: (a) mixing gallium oxide, oxides of rare earth elements, oxide of alkaline earth element, at least one of zinc oxide and magnesium oxide to form a mixture; (b) firing the mixture at a temperature ranging from 1050 to 1200xc2x0 C. for a time ranging from 2 to 10 hours to accomplish synthesizing a compound material; (c) pulverizing the compound material; and (d) compacting the pulverized compound material; and (e) sintering the compacting compound material to form the solid electrolyte.
A further aspect of the present invention resides in a solid electrolyte represented by the following formula:
La(1xe2x88x92xxe2x88x92y)LnxAyGa(1xe2x88x92z)BzO3xe2x88x920.5(x+y+z)
where Ln is at least one element selected from the group consisting of Gd, Sm and Nd; A is Ba; B is Mg; x is 0.1; y is 0.1; and z is 0.2, wherein the solid electrolyte is formed of particles whose means diameter is within a range of from 4 to 10 xcexcm, the solid electrolyte being produced by a method comprising: (a) mixing lanthanum oxide, gallium oxide, oxide of at least one rate earth element selected from the group consisting of Gd, Sm and Nd, barium oxide and magnesium oxide to form a mixture; (b) firing the mixture in air at a temperature ranging from 1100 to 1200xc2x0 C. for a time ranging from 2 to 8 hours to accomplish synthesizing a compound material; (c) pulverizing the compound material; (d) compacting the pulverized compound material; (e) adjusting mean diameter of the pulverized compound material within a range of from 0.5 to 0.8 xcexcm; and (f) sintering the compacting compound material in air at a temperature ranging from 1400 to 1500xc2x0 C. for a time ranging from 2 to 8 hours to form the solid electrolyte.
A still further aspect of the present invention resides in a fuel cell comprising a solid electrolyte represented by the following formula:
La(1xe2x88x92xxe2x88x92y)LnxAyGa(1xe2x88x92z)BzO3xe2x88x920.5(x+y+z)
where Ln is rare earth element; A is at least one element selected from the group consisting of Sir, Ba and Ca; B is at least one of Mg and Zn; x is a number ranging from 0.05 to 0.15; y is a number ranging from 0.05 to 0.15; and z is a number ranging from 0.05 to 0.25.